1. Field of the Invention
This invention relates to an error detecting mechanism for a servosystem, capable of preventing servo malfunctions in a driving motor caused by errors such as a disconnection, short-circuit and disturbance noises in a servo loop system.
2. Description of the Prior Art
As well known, servomotors have been widely utilized in the field of position measurement control and the like, and in order to prevent malfunctions in the servomotors, heretofore, there have been used error detecting mechanisms for servosystems.
FIG. 1 shows the conventional error detecting mechanism provided on joint portions of arms and mechanical hand of an industrial robot, for a driving motor 10 (a DC motor) to drive these joint portions.
As will be described hereunder, the mechanism of the type described comprises a rotary angle detecting system 12 for detecting a rotary angle of a driving motor 10, a driving controlling system 14 for driving and controlling the driving motor 10 and an error preventing system 16 for effecting an emergency stop of the driving motor 10 on servo malfunction of the driving motor 10.
Referring to FIG. 1, the rotary angle detecting system 12 is housed together with the driving motor 10 in a motor unit, and comprises an encoder 18 directly connected to a rotary shaft projecting from behind the driving motor 10 and a counter 20 supplied with an output pulse train 100 outputted from the encoder 18 and counting the same. The counter 20 is reset when an arm of an industrial robot is brought into a reference posture for starting the industrial robot in operation, and thereafter, can count the output pulse train 100 outputted from the encoder 18 during operation of the industrial robot.
Furthermore, the driving-controlling system 14 includes a rotary angle commanding circuit 22 for commanding a rotary angle of the driving motor 10 and a comparator 24 supplied with a relative rotary angle detection value 102 from the counter 20 and a rotary angle command value (a desired rotary angle) 104 from the rotary angle commanding circuit 22 and extracting a deviation value of the relative rotary angle detection value 102 from the rotary angle command value 104, and can drive and control the driving motor 10 in accordance with a deviation value 106 outputted from this comparator 24. More specifically, in this driving-controlling system 14 of this driving motor 10, the deviation value 106 from the comparator 24 is converted into an analogue value by a D/A converter 26 and supplied to a driver 28 to drive the driving motor 10, with the result that the rotary angle of the driving motor 10 is feedback-controlled to a value corresponding to the rotary angle command value 104.
The above-described error preventing system 16 of the conventional mechanism is of such an arrangement that, when the deviation value 106 from the comparator 24 exceeds a preset servo deviation tolerance value (a threshold value), an emergency stop of the driving motor 10 is effected. More specifically, in FIG. 1, the deviation value 106 from the comparator 24 is supplied to an emergency stop control circuit 30 provided in the error preventing system 16 and storing a servo deviation tolerance value 108, and the emergency stop control circuit 30 can output an open-command signal 110 and a close-command signal 112 simultaneously when the deviation value 106 exceeds the telerance value 108. This open-command signal 110 causes a contact 32 to be opened, whereby a current supply to the driver 28 is cut off to stop the driving motor 10 in operation, and the close-command signal 112 causes a contact 34 to be closed, whereby a current (AC) is supplied to a brake 36 provided in the aforesaid motor case, to thereby effect an emergency stop of the driving motor 10.
The conventional mechanism shown in FIG. 1 is of the above-described arrangement. Description will hereunder be given of operation thereof.
As described above, for starting the industrial robot in operation, the arm is brought into the reference posture and the counter 20 is reset. Then, the joint portion of the arm starts rotation from the rotary angle command value 104 as being the initial value, this rotation causes the output pulse train 100 from the encoder 18 to be counted by the counter 20, and the relative rotary angle detection value 102 is supplied to the comparator 24. The deviation value 106 outputted from this comparator 24 acts so as to cause a rotary angle of the driving motor 10 to coincide with the command value 104, to thereby effect a servo control.
When an error occurs in the driving motor 10 during the above-described servo control and the deviation value 106 exceeds the tolerance value 108 preset in the emergency stop control circuit 30, the driver 28 is stopped in operation by the open-command signal 110 and the close-command signal 112, both of which are outputted from the emergency stop control circuit 30 and the brake 36 acts, whereby the driving motor 10 is brought to an emergency stop.
However, the conventional mechanism described above has been monitoring a possible error during the servo control of the driving motor 10 only by use of the output pulse train 100 from the encoder 18. In consequence, if the encoder 18 is thrown into disorder and the feedback (the relative rotary angle detection value 102) to the driving-controlling system 14 is cut off, then a drift of the driver 28 causes the driving motor 10 to begin to rotate, whereby the deviation value 106 is not varied even if an error occurs, so that the emergency stop control circuit 30 cannot detect a malfunction of the driving motor 10. In consequence, there has been a possibility of loss of error detecting function for the driving motor 10, thereby resulting in the disadvantage of lowered reliability.